I would look into ati 4870 the 512 ddr 4 cards are around $230 the ddr 3 are $200. They are supposed to be awesome and you can cross fire it with your 3870 as long as you have 2 16x pci slots.
The ATI Radeon 4870 and ATI Radeon 3870 are not capable of Crossfire together.
"-ATI official-
Despite previous reports on the internet that old Radeon 38x0 and new Radeon 48x0 cards will work in Crossfire together, unfortunately this won't happen.
Two senior software guys at ATI has just confirmed that 3800 series in Crossfire with 4800 won't work and that such a configuration is not supported.
You won't be able to match the generations and we are investigating if you will be able to mix cards from 48x0 series. We believe you should, but we won’t jump the gun with this announcement.
Sorry for destroying one cool dream, but you will have to forget about your 38x0 card and swap it for a much faster 48x0 generation.
At the same time, our friend W1zzard from tech power up has confirmed that he couldn't make 3800 and 4800 generation cards work together in XP, but now we know the answer why. It won't work on XP or Vista, simply as it's not supported. "